Input: document and check on implicitly defined FF_MAX_EFFECTS

There is an undocumented upper bound for the total number of ff effects:
    FF_GAIN (= 96).
This can be found as follows:
- user: write(EV_FF, effect_id, iterations)
    calls kernel: ff->playback(effect_id, ...): starts effect "effect_id"
- user: write(EV_FF, FF_GAIN, gain)
    calls kernel: ff->set_gain(gain, ...): sets gain

A collision occurs when effect_id equals FF_GAIN.
According to input_ff_event(),
FF_GAIN is the smallest value where a collision occurs.
Therefore the greatest safe value for effect_id is FF_GAIN - 1,
and thus the total number of effects should never exceed FF_GAIN.

Define FF_MAX_EFFECTS as FF_GAIN and check on this limit in ff-core.

Signed-off-by: Elias Vanderstuyft <elias.vds@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/drivers/input/ff-core.c b/drivers/input/ff-core.c
index eab56c0aacd5..8f2042432c85 100644
--- a/drivers/input/ff-core.c
+++ b/drivers/input/ff-core.c
@@ -318,6 +318,11 @@ int input_ff_create(struct input_dev *dev, unsigned int max_effects)
 		return -EINVAL;
 	}
 
+	if (max_effects > FF_MAX_EFFECTS) {
+		dev_err(&dev->dev, "cannot allocate more than FF_MAX_EFFECTS effects\n");
+		return -EINVAL;
+	}
+
 	ff_dev_size = sizeof(struct ff_device) +
 				max_effects * sizeof(struct file *);
 	if (ff_dev_size < max_effects) /* overflow */